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Re: heap_tuple_needs_freeze false positive

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: heap_tuple_needs_freeze false positive
Date: 2012-02-02 14:30:48
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Lists: pgsql-hackers
On Wed, Feb 1, 2012 at 8:01 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> Suggested patch attached.  I'd backpatch this as far as it applies
> cleanly.

This is new code in 9.2, but it's modelled on heap_freeze_tuple(), which is old.

I'm not convinced that it's a bug.  Suppose that xmax is set but is
hinted as invalid.  We process the table and advanced relfrozenxid;
then, we crash.  After recovery, it's possible that the hint bit is
gone (after all, setting hint bits isn't WAL-logged).  Now we're in
big trouble, because the next CLOG lookup on that xmax value might not
happen until it's been reused, and we might get a different answer
than before.

Robert Haas
The Enterprise PostgreSQL Company

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